1. Field of the Invention
Aviation safety
2. Description of the Prior Art
Helicopter operations are growing steadily in the aviation field. As their unique capabilities are understood, the demand for their operation has increased. In both military and civilian flying, helicopter operations cannot be conducted under weather conditions conducive to icing without a rotor blade ice protection system. An unprotected rotor blade accretes ice which disturbs the airflow over the blade, reducing its lifting capability. Also the blade flexing tends to break pieces of ice off of the blade which causes an unbalance situation accompanied by extreme vibration of the whole helicopter. Blade protection from ice must be effected in flight through all-weather conditions if the flight is to be successfully and safely conducted.
Over the past 20 years numerous methods have been attempted to provide an ice protection system for a helicopters main rotor blade. None have been wholly satisfactory and no single system is in general use in the United States. Ice protection for helicopter blades has followed the general trend of ice protection for fixed wing aircraft.
The approaches to precluding ice collection on the rotor blade have included heating the leading edge of the blade to a temperature where ice will not form; covering the leading edge of the blade with a material that has a low adhesion for ice; using mechanical means of removal; and using chemicals from within the blade to flow through holes in the leading edge of the blade where they mix with the impinging super cooled droplets to preclude their freezing on the blade.
Hot air has been circulated through the leading edge of a rotor blade during tests, where it was found that the heat required to protect the entire blade was such that the size of the heat generator was too large to be practical, and that heat losses were so great that the outer portion of the blade could not be protected.
Various types of electrical systems are in use at present. Sufficient heat is not available to preclude ice forming, so the ice is allowed to collect, then the electrical system furnishes heat which melts the bond of the ice and the ice is removed by the airflow over the blade. This type of system is complex, expensive, and often requires more electrical power than a helicopter can provide. In addition, special blades are required.
Numerous material, including teflon, have been installed on rotor blades with the hope that the ice adhesion characteristics would be so low that the airflow and the centrifugal forces would shed the ice. None have proven practical.
Mechanical methods have been successful in removing ice from the fixed wings of an aircraft. Here rubber boots are installed on the leading edge of a wing and when sufficient ice is collected, air is forced into the rubber boot which inflates it, cracking off the ice. This system was tried for helicopters, however the high speed of the rotor blade causes mechanical problems with the inflatable boot and no workable system has been devised. A mechanical method of vibrating the wing to break off the ice has been discussed but no flying model has ever worked.
Chemicals have been successfully used on both fixed wing and rotary wing aircraft to prevent ice formations on lifting surfaces - wings and rotor blades. A Russian designed chemical system for helicopters has proven very effective in preventing ice formation on rotor blades. This system uses a freezing point depressant fluid which is pumped into a slinger ring on the rotor head where it is distributed in the blade through a series of tubes from which it oozes through holes on the leading edge of the blade, covering the blade and precluding ice formation. This requires a special blade with a complex interior system that is only necessary when icing flight is contemplated. It is subject to maintenance problems as the exit holes on the leading edge of the blade are continually exposed to impinging air and dust. The installation and removal of this system is not simple.
This invention proposes an ice protection system using the same principals of chemistry for the ice prevention, but uses a different method of coating the blades with the depressant fluid. The primary purpose in devising the present invention is to supply a device that provides ice protection for the main rotor blade of a helicopter that must operate in all-weather conditions.
A further purpose for devising the present invention is to provide an inexpensive ice protection system that can have external elements which can easily be installed or removed and that does not require a special rotor blade, different from those normally used.